The present invention relates to an observer tracking directional display. Such a display may be used as a three dimensional (3D) autostereoscopic display, for instance in 3D television, medical imaging, computer games, telephony, scientific visualisation, virtual reality and office automation equipment.
Observer tracking 3D autostereoscopic displays are disclosed in EP 0 721 131 and EP 0 726 482. These patents are primarily concerned with displays in which observer tracking is achieved in arrangements with no moving parts. However, reference is made to the possibility of using mechanical tracking arrangements.
EP 0 625 861 discloses a spatial light modulator (SLM) of liquid crystal device (LCD) type having a picture element (pixel) configuration which is particularly suitable for use in autostereoscopic displays. In particular, this configuration allows contiguous display windows to be produced and enhanced horizontal display resolution to be achieved. European Patent Application No. 96304959.8 discloses a technique for making SLMs having such a pixel configuration.
xe2x80x9cAn in-cockpit xe2x80x9csituation awarenessxe2x80x9d autostereoscopic avionics displayxe2x80x9d, J. B. Eichenlaub, T. Touris, SPIE vol. 2219 xe2x80x9cCock-pit Displaysxe2x80x9d 1994 pages 395 to 406 discloses an observer tracking system for a flat panel display in which viewing windows are switched between discrete positions by switching a light source. However, such an arrangement has a number of disadvantages. For instance, switching takes place so that one light source has to be switched off while another is switched on. If this is not done accurately or if there is a decay of the phosphors of the lamps, the display will appear to become brighter or darker during switchover, thus causing significant image flicker. Also, the intensities of the light sources must be matched in order to avoid unwanted image flicker which would be visible as an observer moved position. Further, if a large number of individual positions is required, then a large number of lamps will be required, thus increasing the display cost and-bulk. Additionally, an arrangement for mechanically translating a lenticular screen to allow observer tracking is disclosed.
EP 0 404 289 discloses a 3D display in which an observer is tracked by moving a curved lenticular screen with respect to an image display. The lenticular screen can be moved laterally and longitudinally so as to track observer movement. However, no details of tracking are disclosed.
xe2x80x9cEye-position tracking stereoscopic display using image shifting opticsxe2x80x9d, H. Imai et al, SPIE vol. 2653, (1996) pages 49 to 55 discloses an arrangement in which an image is projected onto the rear of a lenticular screen and is tracked with respect to the screen by moving the projection optics in accordance with the measured position of an observer.
U.S. Pat. No. 5,264,964 discloses a display which is switchable between autostereoscopic and stereoscopic modes of operation by means of mechanical movement.
A known type of autostereoscopic 3D display uses a single display panel, such as a liquid crystal device spatial light modulator, on which several two dimensional (2D) reviews are spatially multiplexed. Vertical slices of the 2D views are interleaved and a parallax optic is used to allow the views to be seen in the intended directions. This creates xe2x80x9cviewing windowsxe2x80x9d as described in more detail hereinafter. The spatial resolution, especially the lateral or horizontal resolution, of the panel has to be shared among the 2D views so that, for N 2D views, each view is displayed with a (horizontal) resolution of Rxc3x97N, where R is the (horizontal) spatial resolution of the panel. For a large number of 2D views, the (horizontal) resolution has to be sacrificed and/or a panel of higher (horizontal) resolution and hence cost must be used.
Some observer tracking displays of this type require that the viewing windows be repeated in several lobes. This requires that the optical performance, such as aberrational and diffractional performance of the display, particularly of the parallax optic, be sufficient to avoid undesirable visual artefacts from being visible in non-zero lobes.
Some observer tracking displays of this type require good window performance. For instance, to avoid visible variations in intensity or flicker as the observer moves, the intensity may vary laterally across a large part, or even all, of the window by only a relatively small amount. Also, the window edge performance may be critical to avoiding undesirable visual artefacts. For instance, it may be necessary for the windows to be accurately contiguous with minimal overlap and underlap. This can place stringent physical requirements, for instance on the manufacturing tolerances of the components such as the panel and on the accuracy of assembly and alignment of the components.
For some applications, mechanical tracking systems, ie: tracking system with moving parts, may have advantages over non-mechanical systems, ie: systems without moving parts. For instance, in mechanical tracking systems, it is possible to use only two viewing windows in a single lobe, such as the zero order lobe where optical performance is best. For spatially multiplexed single panel displays, this minimises the loss of resolution for each 2D view compared with the spatial resolution of the panel so that lower resolution (and less expensive) panels may be used or the resolution of the 3D image may be improved. The use of only one lobe allows the optical performance requirements of the parallax optic to be relaxed so that a less expensive parallax optic may be used. Because the windows track the observer position, contiguity of the windows may not be necessary and low variation in intensity may be required across only a smaller part of each window. This reduces the physical requirements and the cost of the display.
Although mechanically tracked systems have been suggested, for instance as mentioned hereinbefore, the practical problems of realising such systems have not previously been considered or addressed. For instance, all mechanical tracking systems will have inertia. Thus, although the delays involved in measuring the observer position will be common to all observer tracking displays, the time between detecting a new observer position and causing the display to move the viewing windows to the desired new position is inherently longer for a mechanically tracked system than for a non-mechanical system. Also, there will be backlash in many types of mechanical system resulting in reduced accuracy of positioning of the viewing windows.
According to the invention, there is provided an observer tracking directional display comprising an image display, a parallax device co-operating with the image display to define at least one viewing zone from which the image display is visible, and an observer tracker for determining the position of an observer, characterised by an electromechanical system responsive to the observer tracker for moving the parallax device relative to the image display to any one of a plurality of discrete stationary positions so that the viewing zone tracks the position of the observer.
The mechanical system may be arranged to provide, at least for an observer at a predetermined longitudinal distance from the display, a number n of discrete stationary positions of viewing windows per interocular distance e given by:
e/nxe2x89xa1Vmax.t+xcex94x
where Vmax is the maximum lateral observer speed for the display, xcex94x is the lateral position error of the observer tracker and t is the time delay between measurement of an observer position and completion of relative movement between the parallax device and the image display to track an observer movement.
The electromechanical system may be arranged to provide, for an observer at at least one longitudinal distance from the display different from the predetermined distance, a number nxe2x80x2 of discrete stationary positions of viewing windows per interocular distance greater than n.
The display may be for three dimensional autostereoscopic viewing, the image display may be arranged to display spatially multiplexed left and right images and the parallax device may be arranged to co-operate with the image display to form left and right viewing windows for left and right eyes, respectively.
The image display may comprise a plurality of picture elements, each of which is of substantially constant vertical aperture across its horizontal aperture. Each of the picture elements may be of substantially rectangular shape. The picture elements may be arranged as pairs of columns with the columns of each pair being substantially horizontally contiguous. The pairs of columns may be spaced apart horizontally.
The image display may comprise a spatial light modulator and a backlight. The spatial light modulator may comprise a liquid crystal device.
The parallax device may comprise a lens array. The lens array may comprise a lenticular sheet.
The parallax device may comprise a hologram array.
The parallax device may comprise a parallax barrier. The parallax barrier may be switchable to a state of uniform transparency for switching the display to a non-directional mode.
The parallax barrier may comprise first and second sheets which are laterally relatively moveable between first and second relative positions, the first sheet comprising a plurality of first transmissive strips separated by first patterns of transmissive and non-transmissive regions, the second sheet comprising a plurality of second transmissive strips separated by second patterns of transmissive and non-transmissive regions which are complementary to the first patterns and arranged such that, in the first relative position, the first strips are aligned with the second strips and the non-transmissive regions of the first and second patterns are aligned with the transmissive regions of the second and first patterns, respectively, and such that, in the second relative positions, the first and second transmissive strips are aligned with the first and second patterns, respectively, and the transmissive regions of the first and second patterns are aligned with each other.
The parallax barrier may comprise superimposed first and second sheets which are laterally moveable between first and second relative positions, the first sheet comprising a plurality of first transmissive strips, each pair of which is separated by alternate strips of first and second orthogonal polarisations, the second sheet comprising a plurality of second transmissive strips, each pair of which is separated by alternate strips of the second and first polarisations such that, in the first relative position, the first transmissive strips are aligned with the second transmissive strips and the strips of the first and second polarisations of the first sheet are aligned with the strips of the second and first polarisations, respectively, of the second sheet and such that, in the second relative position, the strips of the first and second polarisations of the first sheet are aligned with the strips of the first and second polarisations, respectively, of the second sheet.
The image device may be arranged to supply light which is polarised in a first direction and the parallax barrier may comprise: a patterned polarisation rotator having first slit-shaped regions separated from each other by second regions, the first and second regions being arranged to supply light of orthogonal first and second polarisations, respectively; and a polariser arranged to transmit light of the first polarisation and substantially to block light of the second polarisation.
The image display may be a light-emissive display.
The separation of the viewing windows corresponding to adjacent ones of the discrete stationary positions may be greater than the accuracy of the observer tracker.
The electromechanical system may comprise an electromechanical transducer connected by a mechanical transmission to the image display or the parallax device. In a first embodiment, the transducer comprises a stepper motor. In a second embodiment, the transducer comprises a servo motor. In a third embodiment, the transducer comprises a voice coil stage. The electromechanical system may comprise a relative movement step encoder. The electromechanical system may comprise a relative position reference detector.
The applicant is the first to realise that, although continuous tracking systems in which the window positions can be continuously adjusted are possible, at least in theory, they would involve certain intrinsic problems when reduced to practice. In order for an observer tracking display to function correctly, the system for detecting the observer position must be correctly interfaced with the system for moving the windows so that the windows track the observer with sufficient accuracy. The signals used to drive the window moving system may be derived from the observer position data by a calculation technique or by a calibration technique. The calculation technique calculates, for each new observer position, the required movement of the moving parts of the optical system and is based on a mathematical model of the whole display. For this technique to provide sufficiently accurate window positioning with a sufficiently small time delay so that the observer can be satisfactorily tracked, a substantial amount of processing power is necessary which gives rise to substantial cost and complexity.
The required processing power can be reduced by using the alternate calibration technique, for instance as disclosed in EP 0 769 881. In this case, the display is calibrated at a plurality of calibration points. For instance, an observer or a sensor moves to a set of positions evenly distributed about the viewing region of the display and, at each position, the viewing windows are controlled manually or automatically to be in the optimun location for viewing the display from that position. The position signal from the observer position measuring system is then associated with the control signals to the mechanical display tracking system, for instance in a look-up table. If a sufficiently large density of calibration points is provided throughout the viewing region, observer tracking may be performed simply by using a look-up operation which requires minimal calculation and hence minimal processing power. However, to obtain the required density throughout an adequate viewing region, a very large number of calibration points is required so that a large and hence expensive memory capacity must be provided. Also, a large number of calibration points makes the calibration time-consuming and hence increases the expense. The memory requirement and the calibration time may be reduced by substantially reducing the density of calibration points and performing interpolation for observer positions away from the calibration points. However, this substantially increases the processing requirement with the disadvantages mentioned hereinbefore.
The applicant is also the first to realise that the number of discrete window positions can be optimised. Too many window positions require better optical performance of the display, for instance in terms of the aberrational or diffractional performance of the parallax device. Too many positions leads to more calibration points being required. Thus, by optimising the number of window positions with respect to the maximum observer speed and observer position measurement error, acceptable performance can be provided at minimum cost.
It is thus possible to provide an electromechanically tracked display using moving parts which are moved in a stepwise manner and, for instance, using pixel configurations of the type disclosed in EP 0 625 861. Accurate lateral contiguity of pixels is not essential so that, for instance, an LCD panel may be used having less stringent tolerance requirements. The display is tolerant of backlash in the moving parts, such as in a mechanical transmission, and allows reduced resolution, accuracy and repeatability of the tracking system. Relatively large system lags or delays can be tolerated and a relatively simple translation control system may be used. Thus, a display of reduced cost and increased robustness can be provided.
Such a display exhibits reduced intensity fluctuations as an observer moves laterally with respect to the display. The display is tolerant of a larger range of interocular separations of observers and provides increased lateral and longitudinal viewing freedom.